Broadband high power light source

ABSTRACT

The present invention relates to a super continuum light source comprising a pump source arranged to emit light having a center wavelength λ center  arranged to provide pump pulse to a generator fibre, where the refractive index profile of the core is arranged to allow modal cleaning of the light is it propagates, such as via stimulated Raman scattering. An example of invention is the application of a relatively high power pump laser utilized to provide an optical super continuum with relatively high spectral density and/or good beam quality even though the pump laser may provide a beam with a high M 2 .

FIELD OF INVENTION

The present invention relates to a super continuum light sourcecomprising a pump source arranged to emit light having a centrewavelength λ_(centre) and a spectral width Δλ, the light source furthercomprising a generator fibre, said generator fibre having an input endand an output end, the latter being arranged to emit an optical supercontinuum when the light source is in use.

BACKGROUND ART

Super continuum (SC) generation is a nonlinear phenomenon characterisedby dramatic spectral broadening of intense light passing through anonlinear material. SC generation occurs in various media and may finduse in numerous applications such as spectroscopy or ultra-short-pulsegeneration. Spectral slicing of a generated SC is an elegant way ofreplacing multiple light sources having separate wavelengths.

Many prior art SC light sources are based on micro structured fibres(MF) which posses unique optical properties with a large degree offreedom of design thus allowing for optimization of the fibre for SCgeneration. However, SC generation in standard fibre has also beendemonstrated. Most prior art disclosures of SC sources utilisefemto-second (fs) pulses (10⁻¹⁵ s) to generate the SC. The physicalmechanism responsible for the SC generation is believed to be due tomultiple nonlinear processes. It has also been shown that it is possibleto create SC by use of pico- and nanosecond pulses, and the mechanismresponsible for these SCs is often attributed to a combination of fourwave mixing and stimulated Raman scattering. In either case of pumpduration and/or fibre type, the size of the guiding core has beenrelatively small (<6 μm diameter) to ensure a high optical density inthe core so that non-linear effects are maximized. This limits theoptical power with which the fibre may be pumped either due to thedamage threshold of the end facet of the fibre or the damage thresholdof the fibre core. The optical power may also be limited by theavailability or cost of a high power laser having an output with asufficient beam quality to allow a substantial amount of its output toenter the fibre.

DISCLOSURE OF INVENTION

Many applications of SC light sources may benefit from high opticalpower particularly when a relatively high beam quality of the SC may beobtained. In this way each slice of the spectrum may in one embodimentperform substantially as a laser with relatively high power.Accordingly, in one embodiment the present invention relates to a SClight source capable of providing an output with relatively high opticalpower.

In one embodiment the invention relates to a SC light source comprisinga pump source arranged to emit pump light in a pump beam, said pumplight having a centre wavelength λ_(centre), the light source furthercomprising a generator fibre having an input end and an output end, saidgenerator fibre being arranged to support a fundamental mode, a zerodispersion wavelength λ_(ZDW) for said fundamental mode and a transverserefractive index profile with a core region having a characteristicdiameter D≧10 μm, said pump source being arranged to inject pump lightinto the input end of said generator fibre with an optical peak powerP≧100 W and a spectral width Δλ spectrum, wherein said refractive indexprofile is arranged to allow SRS modal cleaning andλ_(ZDW)−(λ_(centre)+Δλ/2)≧0 such as ≧0.5 THz·λ² _(centre)/c, such as ≧2THz·λ² _(centre)/c, such as ≧5 THz·λ² _(centre)/c, such as ≧10 THz·λ²_(centre)/c, such as ≧13 THz·λ² _(centre)/c, such as ≧26 THz·λ²_(centre)/c, such as ≧39 THz·λ² _(centre)/c, such as ≧52 THz·λ²_(centre)/c, such as ≧65 THz·λ² _(centre)/c, such as ≧78 THz·λ²_(centre)/c, such as ≧−91 THz·λ² _(centre)/c where c is the speed oflight in the core. In one embodiment said generator fibre supports oneor more modes at said centre wavelength. In one embodiment the inventionrelates to a system comprising such a light source.

The large core of the generator fibre may accept pump light with a highoptical peak power and/or high average power without reaching break downlimits of the end facet and/or bulk material of the core. In oneembodiment a relatively high numerical aperture (NA) combined with therelatively large core allows the core to accept pump beams withrelatively high M². The inventors have found that by utilizing so calledSRS modal cleaning (discussed below) red shifted pump light (e.g. via aRaman shift) will have increased power in the fundamental mode of thefibre relative to pump light in higher order modes after this redshifting. At the same time the inventors have realized that pump lightat longer wavelengths than the zero dispersion wavelength may initiatethe generation of an often wide smooth super continuum. The inventorshave surprisingly found that when this smooth SC is generated due topump light having a low number of modes or being substantially singlemode the generated super continuum will maintain this beam quality.However, if the generator fibre is pumped with multimode light directlyat a wavelength longer than λ_(zow) the generated SC will likely inheritthe multimode nature of the pump light. Note that throughout thisspecification at least in one embodiment wavelength refers to the centrewavelength of the pump light. The fibre is in one embodiment at leastpartly pumped below λ_(zow) to allow SRS model cleaning to take effectbefore the pump light red-shifts past λ_(ZDW). The term “partly” herereferring to that some of the pump light may in one embodiment be abovea ZDW of the fibre. Once red-shifted past the λ_(ZDW) the red-shiftedpump light will in one embodiment have lower M² values which aresubstantially transferred and maintained by the SC generated from thislight. This smooth SC will often also contribute at wavelengths lowerthan λ_(ZDW) likely due to phase-matching conditions being fulfilledbetween lower and higher wavelengths. In one embodiment the inventionrelates to a method of providing a high power super continuum with highbeam quality comprising

-   -   a. Providing a generator fibre having an input end and an output        end and a zero dispersion wavelength λ_(ZDW) for a fundamental        mode and a transverse refractive index profile with a core        region having a characteristic diameter D≧10 μm    -   b. Providing a pump light source arranged to emit pump light in        a pump beam, said pump light having a centre wavelength        λ_(centre) and an optical peak power P≧100 W and a spectral        width Δλ spectrum    -   c. Arranging said pump source to inject pump light into the        input end of said generator fibre        said generator fibre being arranged to support a fundamental        mode at least at said centre wavelength wherein said refractive        index profile is arranged to allow SRS modal cleaning and        λ_(ZDW)−(λ_(centre)+Δλ/2)≧0 such as ≧0.5 THz·λ² _(centre)/c,        such as ≧2 THz·λ² _(centre)/c, such as ≧5 THz·λ² _(centre)/c,        such as 100 THz·λ² _(centre)/c, such as ≧13 THz·λ² _(centre)/c,        such as ≧26 THz·λ² _(centre)/c, such as ≧39 THz·λ² _(centre)/c,        such as ≧52 THz·λ² _(centre)/c, such as ≧65 THz·λ² _(centre)/c,        such as ≧78 THz·λ² _(centre)/c, such as ≧91 THz·λ² _(centre)/c        where c is the speed of light. In one embodiment the fibre        further supports one or more higher order modes at least at said        centre wavelength.

The red shifting along with other non linear process occur as the lightpropagates through the generator fibre. The modal quality of the lightin the fibre may therefore change with position. In the present contextthe M² _(in) side the fibre is in one embodiment taken to mean the M²value of the beam due to the light had it been allowed to escape fromthe fibre through an end facet located at that position along the fibre.

In the present context a super continuum spectrum is in one embodimenttaken to mean a spectrum spanning more than one octave, such as morethan 1.5 octave, such as more than 2 octaves, such as more than 3octaves, such as more than 4 octaves, such as more than 5 octaves. Inone embodiment a super continuum is taken to mean a spectrum of lighthaving spectral width of more than 500 nm, such as more than 700 nm,such as more than 800 nm, such as more than 900 nm, such as more than1000 nm, such as more than 1100 nm, such as more than 1200 nm, such asmore than 1300 nm, such as more than 1400 nm, such as more than 1500 nm,such as more than 1600 nm, such as more than 1700 nm, such as more than1800 nm, such as more than 1900 nm, such as more than 2000 nm. In oneembodiment a reference to a super continuum light source is taken tomean source that the pump source and said generator fibre are arrangedso that during use said generator fibre emits output having such aspectrum. A super continuum may have valleys, peaks and holes in thespectrum. In this context spanning is taken to mean that the spectralpower is more than −20 dB/nm relative to the mean optical power of theoutput light, such as more than −10 dB/nm, such as more than −5 dB/nm,such as more than −3 dB/nm, such as more −2 dB/nm in more than 50% ofthe span, such as in more than 75% of the span, such as in more than 85%of the span, such as in more than 95% of the span. In one embodimentspanning is taken to mean that the spectral power is more than −20dBm/nm, such as more than −10 dBm/nm, such as more than −5 dBm/nm, suchas 0 dBm/nm, such as more than 5 dBm/nm, such as more than 10 dBm/nm,such as more than 20 dBm/nm, such as more than 30 dBm/nm, such as morethan 40 dBm/nm in more than 50% of the span, such as in more than 75% ofthe span, such as in more than 85% of the span, such as in more than 95%of the span.

It is noted that in the present context a pump source may in oneembodiment comprise a laser followed by one or more amplifiers arrangedto raise the power level. In one embodiment the pump source comprisesone or more optical components arranged to shape the spectrum of thepump light. In one embodiment such amplifier(s) and/or opticalcomponents are understood to be a part of the pump source although nospecifically mentioned.

The light emitted at the output end of the fibre when the light sourceis in use, otherwise referred to as output light, will commonly compriseone or more of: attenuated components from the pump source otherwisetransmitted through the fibre substantially unchanged, Raman-shifted (orotherwise red-shifted) pump light at shorter wavelengths than λ_(ZDW),and a SC generated by red-shifted pump light having wavelengths longerthan λ_(ZDW). In one embodiment the red shifted pump light may in itselfbe said to form (often a peaked) SC. In the context of the present textwe refer to the SC generated from red shifted pump light at wavelengthslonger than λ_(ZDW) as the smooth SC (although this may not necessarilybe smooth) and light generated from one or more red shifts of lighthaving a wavelength below λ_(ZDW) as red shifted pump light. The sum ofthese components at the output end of the fibre is referred to as thegenerated SC. The output light may further comprise attenuated pumplight otherwise transmitted through the generator fibre unchanged.

In one embodiment the inventors have surprisingly found that the smoothSC in combination with the red shifted pump light forms output lightwith low values of M². Accordingly, a super continuum light sourceaccording to the invention may in one embodiment provide output lightwith a widely spanning SC having a considerable spectral density as thefibre allows pump light with considerable peak power. At the same timethe light source may in one embodiment provide a beam with high quality(i.e. low M²). This may in several applications dramatically increasethe utility of the high power SC output.

In the context of the present text spectral width of the output lightisin one embodiment taken to mean the widest measure of the spectrumwherein the end points are at least at −50 dB power per nm relative tothe power per nm at the center wavelength of the pump source, such as atleast at −20 dB per nm, such as at least at −10 dB per nm, such as atleast at −5 dB per nm, such as at least at −3 dB per nm. In the eventthat the pump source provides two or more separate center wavelengthsthis relative power measurement is in one embodiment to be measuredrelative to the center wavelength having the lowest power in the outputlight. In one embodiment the relative power measurement is measuredrelative to the center wavelength having the highest power in the outputlight. In one embodiment the relative power measurement is in oneembodiment to be measured relative to the mean of the power of theoutput light at each center wavelength. In one embodiment the injectedpump light is in itself a super continuum. In one such an embodiment therelative power measurement is to be measured relative to the maximumpower per nm of the output light. In one such an embodiment the relativepower measurement is to be measured relative to the mean power per nm ofthe output light where the mean is calculated over all wavelength havingmore than −30 dB/nm relative to maximum power per nm of the spectrum,such as more than −20 dB/nm, such as more than −10 dB/nm.

In one embodiment the invention relates to a tuneable light sourcecomprising a light source according to the invention and a non-linearcrystal arranged so that at least a part of output light emitted fromthe output end of the generator fibre is brought into interaction withsaid non-linear crystal under an angle cp relative to the surface ofsaid non-linear crystal. In this way a tunable light source which isable to provide shorter wavelengths than provided by the super continuumlight source may be provided. Compared with prior art super continuumsources, the super continuum may now have sufficient optical power toallow a spectral slice to be frequency doubled, tripled or otherwisefrequency increased by a non linear crystal and obtain relatively highoutput power. In one embodiment the non-linear crystal is selected fromthe group of BBO (β-barium borate), KDP (potassium dihydrogenphosphate), KTP (potassium titanyl phosphate), and lithium niobate. Inone embodiment a tunable light source is obtain via a differentnon-linear material and/or medium.

In one embodiment the invention relates to hyperspectral imaging systemcomprising a light source according the invention. Due to the relativelyhigh spectral density (i.e. optical power per nm wavelength) over a widerange of wavelength, available in one embodiment, the performance ofsystem may be improved as imaging can for example be done remotely, i.e.at a larger distance than lower power hyperspectral imaging systems.This can for example be an advantage for air-borne imaging systems.

In one embodiment the invention relates to a white light LIDAR (DOAS)system comprising a light source according to the invention. Due to thehigh spectral density available in one embodiment over a wide range ofwavelength the performance of the system may be improved as the range ofa LIDAR/DOAS system and/or the resolution of a LIDAR/DOAS system may beenhanced.

In one embodiment the invention relates to an optical amplifiercomprising an active amplifier fibre and a supercontinuum light sourceaccording to the invention arranged to pump said active fibre.

In one embodiment the invention relates to a system for illuminationcomprising a light source according to the invention. Due to therelatively high spectral density available in one embodiment over a widerange of wavelength the performance of system may be improved as therange and/or the resolution and/or the spectral bandwidth and/orspectral position of an illumination system can be improved.

DETAILS OF THE INVENTION

The characteristic diameter of the core is term applied in the contextof the present specification as generalization that enablesdetermination of the diameter of the core even if the core of the fibreis not circular. As is well-known in the art an optical fibre guideslight along its length primarily in the core of the fibre. The core isnormally located along the centre axis of the fibre and is surrounded byone or more cladding regions. Other shapes of cores are also possiblesuch polarization maintaining fibres with an elliptical core. In oneembodiment the fibre comprises two or more cores. In one embodimentthese cores are coaxial cores and such cores may in one embodimentprovide functions well-known in the art such as dispersion control. Inone embodiment the core is not coaxial such as in a dual-core opticalfibre. The cladding region(s) is/are often further surrounded by one ormore coatings and/or other layers often suitable for providingenvironmental and/or mechanical shielding. Light is normally guided inthe core by refraction and/or total internal reflection due to a higherrefractive index in the core relative to the cladding. In one embodimentlight is guided by so-called photonic bandgap (PBG) effect. The averagerefractive index of the core and/or cladding(s) may be engineered bydoping the base material and/or by introducing microstructures runningalong the length of the fibre. Such microstructures are typicallyair/vacuum filled but may also comprise liquids, gasses and solids suchas doped or pure silica. Fibres comprising microstructures (such asphotonic crystal fibres, photonic bandgap fibres, leaky channel fibres,holey fibres etc.) are in this context referred to as microstructuredfibres. Unless otherwise noted the refractive index refers to theaverage refractive index which in one embodiment may be calculated forthe core and each layer surrounding separately it whether the fibre isstandard fibre, where the core and any cladding layers surrounding thatcore have a substantially homogeneous refractive index, or amicrostructured fibre where the core and/or one or more cladding layerscomprise microstructures. In one embodiment the refractive index varieswithin one or more of the core and/or cladding regions such as due to avariation in dopant concentration and/or the fraction ofmicrostructures. In one such embodiment the refractive index refers to alocal average such as a running average In one embodiment the average astaken over a characteristic length corresponding to wavelength, i.e.such as 2 times the wavelength, the wavelength or half of thewavelength.

In microstructured fibres the microstructures in the cladding may forcertain designs be utilized to guide light using the so called photonicbandgab effect which is well-known in the art.

The core and/or cladding of the fibre is in one embodiment composed bysilica as a base material, such as fused silica, with or without dopantsand/or microstructures formed by air/vacuum filled holes. The presentinvention is not necessarily limited to silica based materials; however,for simplicity the remaining presentation of the invention will assume asilica base fibre (standard or micro structured). Changes to parametersor the like due to different base materials will be apparent to askilled person such as the magnitude of a Raman frequency shift or breakdown limits of the material.

In a given cross section perpendicular to the longitudinal axis of afibre, said fibre may be said to have a transverse refractive indexprofile. In one embodiment the cross section of the generator fibre issubstantially constant along the length of the fibre, but in oneembodiment the transverse cross section changes between the input endand the output end. As an example such change may be due to taperingeither reducing or increasing the diameter of the fibre. Such taperingmay also form one or more waists. In one embodiment a reduction of thediameter of the fibre alters the local non linear properties of thegenerator fibre and thus influences the output light of the source. Inone embodiment a variation in the refractive index profile may beapplied to shape the dispersion profile of the fibre. In one embodimentthe location along the fibre of a variation may be chosen according tothe spectrum of the light within the fibre at this location. In oneembodiment a variation in the refractive index profile is due to atapering performed during drawing of the fibre. In one embodiment avariation in the refractive index profile is due to a tapering performedpost to drawing of the fibre. In one embodiment variation in therefractive index profile contributes to extending the spectrum of theoutput light to shorter wavelengths, such as a wavelength below 470 nm,such as below 450 nm, such as below 420 nm, such as below 400 nm, suchas below 390 nm, such as below 380 nm, such as below 370 nm, such asbelow 360 nm, such as below 350 nm. In one embodiment extending theoutput light to shorter wavelengths by tapering is performed accordingto the teachings in the paper “Dispersion decreasing photonic crystalfiber for UV-enhanced supercontinuum generation”, by A. Kudlinski, A. K.George and J. C. Knight, 2006 Digest of the LEOS Summer TopicalMeetings. In one embodiment extending the output light to shorterwavelengths by tapering is performed according to the teachings inpapers, such as Alexandre Kudlinski and Arnaud Mussot, “Visiblecw-pumped supercontinuum,” Opt. Lett. 33, 2407-2409 (2008), and/or J. C.Travers, A. B. Rulkov, S. V. Popov, J. R. Taylor, A. Kudlinski, A. K.George, and J. C. Knight, “Multi-Watt Supercontinuum Generation from 0.3to 2.4 μm in PCF Tapers,” in Conference on Lasers andElectro-Optics/Quantum Electronics and Laser Science Conference andPhotonic Applications Systems Technologies, OSA Technical Digest Series(CD) (Optical Society of America, 2007), paper JTuB2, and/or White lightsupercontinuum: Power struggle (Photonics Spectra, pp. E4-E5, February2010).

In one embodiment the transverse refractive index profile comprises atleast one core region arranged to guide the light. Assuming, for oneembodiment, a cylinder symmetrical core region the refractive indexvariation of the core region may be described as

n(r)=Δn(r)+n _(c),

where n is the refractive index, r is the radial distance from thecentre of the cross section, Δn is the variation of the refractive indexrelative to the cladding refractive index n_(c). In one embodiment thecladding refractive index n_(c) is the average refractive index of thecladding. In one embodiment the cladding refractive index n_(c) is therefractive index at a radial distance, r_(c), outside which less than10% of the fundamental mode of the light propagates, such as less than5% propagates outside r_(c), such as less than 1% propagates outsider_(c), such as less than 0.1% propagates outside r_(c). In oneembodiment the radial distance r_(c) is found for the centre wavelengthof the pump source. In one embodiment r_(c) is found as the largestdistance satisfying the above percentile constrain for the wavelengthswhich the light source is arranged to produce. In one embodiment r_(c)is found as the largest distance satisfying the above percentileconstrain for the wavelengths guided by the generator fibre.

In one embodiment r=r_(c) defines the core region of the generatorfibre.

The core of the generator fibre may be said to have a characteristicdiameter D. In one embodiment D is defined as 2·r_(c). In one embodimentn(r) is substantially at its maximum in the centre of the cross sectionand D is defined as equal to two times the radial distance for whichn(r)=n(0)/2. In one embodiment n(r) is not at its maximum at the centre.In one such embodiment D is defined as two times the maximum distance(less than r_(c)) for which n(r) equals to n_(max)/2.

In an embodiment, the core may comprise co-dopants that provide apredetermined refractive index profile of the core. In an embodiment,the predetermined index profile may have a maximum in the centre of thecore, such as a parabolic index profile, a W-profile, or another profileknown from conventional optical fibres. In one embodiment, thepredetermined index profile may have a local minimum in centre of thecore—such as a local minimum that may result from fabrication of dopedrods using modified chemical vapour deposition (MCVD).

The above discussion of r_(c), n(r) and D has assumed a cylindersymmetry around the centre of the cross section. In the event that thesymmetry is shifted from the centre of the cross section to anothercentre of symmetry, r may be measured relative to this centre providingfor mutatis mutandis changes to the determination of r_(c), n(r) and D.In the event that the transverse refractive index profile is notrotational symmetric, such as a generator fibre with an elliptical coreregion, the transverse refractive index profile may in general bedescribed by a function n(r,θ)=Δn(r,θ)+n_(c)(θ) where θ is an angleranging from 0 to 2λ. In one embodiment the n_(c)(θ) may be assumedsubstantially constant. In one embodiment r_(c) is also a function of rand θ. In one embodiment the origin of the coordinate system defined byr and θ is located in the centre of the cross section. In one embodimentthe origin of the coordinate system defined by r and θ is located in thecentre of gravity of the transverse refractive index profile. In oneembodiment the characteristic diameter D(θ) is generally a function of θdetermined as described above for the cylindrically symmetric case foreach value of θ. In one embodiment the characteristic diameter D is saidto be the minimum value of D(θ). In one embodiment the characteristicdiameter D is said to be the maximum value of D(θ).

In one embodiment the characteristic diameter D is said to be the meanvalue of D(θ).

In one embodiment the generator fibre has a core region substantiallyshaped according to a power-law index profile characterized by

${n(r)} = \left\{ \begin{matrix}{n_{1}\sqrt{1 - {2{\Delta \left( {r/r_{cladding}} \right)}^{\alpha}}}} & {r \leq r_{cladding}} \\n_{cladding} & {r > r_{cladding}}\end{matrix} \right.$

where

${\Delta = \frac{n_{1}^{2} - n_{cladding}^{2}}{2n_{1}^{2}}},$

n₁ is the nominal refractive index on the centre axis, n_(cladding) isthe refractive index of the cladding, which is assumed constant and α isa parameter that defines the shape of the profile hence this profile issometimes called an alpha profile. However, in other embodiments theindex of the cladding may be a function of r and/or θ without departingfrom the power-law index profile of the core. Similar to other types offibres, a layered or otherwise structured cladding may provide forfunctionality of the fibre such as those known from double-clad fibres,air-clad fibres and dispersion compensation fibres.

In one embodiment α=2 and the fibre is said to have a parabolic indexprofile. The refractive index profile for a graded-index fibre iscommonly nearly parabolic. A parabolic profile may result in continuousrefocusing of the light in the centre of the core as the light travelsin the core. A parabolic profile may therefore reduce modal dispersionrelative to a step index fiber. For a step index fibre it may in oneembodiment be said that α→∞.

Relative to the above discussion of refractive index profile it may benoted that real life fibre often has an index profile that deviates fromthe mathematical expression at least as a result of productioninaccuracies. In one embodiment the above mathematical expressions aretaken to describe the refractive index profile to accuracy within 50%,such as within 30%, such as within 15%, such as within 10%, such aswithin 10%, such as within 5%, such as within 3%, such as within 1%. Itis further noted that the above equations assume a single homogeneouscladding surrounding the core. In one embodiment the core is surroundedby one or more cladding layers. The index profile of these claddinglayers may be homogeneous or may vary with r and/or θ. In one embodimentthe fibre comprises a cladding having a power law profile havingsubstantially identical values of α. In one embodiment the fibrecomprises a cladding having a power law profile as well as a core havinga power law profile wherein the α-value of the two is not identical. Inone embodiment one cladding layer acts as cladding (i.e. confines thelight) for some wavelengths whereas it contributes to guiding light atother wavelengths. In such designs the dispersion of the fibre may beshaped which in turn may shape the spectrum such as phase matching toshorter wavelengths. For fibres with multiple claddings and/or cores thecharacteristic diameter may be determined according to the generalcriteria previously discussed.

In one embodiment the characteristic diameter D is larger than or equalto 10 μm, such as larger than or equal to 25 μm, such as larger than orequal to 50 μm, such as larger than or equal to 75 μm, such as largerthan or equal to 100 μm, such as larger than or equal to 125 μm, such aslarger than or equal to 150 μm, such as larger than or equal to 175 μm,such as larger than or equal to 200 μm, such as larger than or equal to250 μm, such as larger than or equal to 300 μm, such as larger than orequal to 350 μm such as larger than or equal to 400 μm such as largerthan or equal to 500 μm such as larger than or equal to 600 μm such aslarger than or equal to 700 μm such as larger than or equal to 800 μmsuch as larger than or equal to 900 μm, such as larger than or equal to1 mm. In one embodiment the characteristic diameter D is less than orequal to 1 mm, such as less than or equal to 900 μm, such as less thanor equal to 800 μm, such as less than or equal to 700 μm, such as lessthan or equal to 600 μm, such as less than or equal to 500 μm, such asless than or equal to 400 μm, such as less than or equal to 350 μm, suchas less than or equal to 300 μm, such as less than or equal to 250 μm,such as less than or equal to 200 μm, such as less than or equal to 175μm, such as less than or equal to 150 μm, such as less than or equal to125 μm, such as less than or equal to 100 μm, such as less than or equalto 75 μm, such as less than or equal to 50 μm, such as less than orequal to 25 μm. A large core may allow for a higher damage threshold ofthe facet of the input end of the fibre and/or a higher damage thresholdof the bulk core as the energy may be distributed over a larger areaand/or volume. On the other hand a core with a smaller core may have ahigher non linearity. In one embodiment the damage threshold of theinput of the generator fibre is increased by splicing, or otherwisemating, the facet with a piece of bulk material, such as a materialsubstantially identical to that forming the core. In this way the inputbeam may be focused inside this material while the transition from airto material is located away from the focal point (typically located onthe fibre facet). Therefore the focal point of the input beam may belocated in bulk material where the break down power is higher.

The generator fibre is in one embodiment multimode at least atwavelengths occupied by the pump light. In one embodiment this allowsfor a higher acceptance of pump light. In one embodiment the so called Vnumber is used to determine whether a fiber is single mode or multimode.The V number is defined as

$V = {\frac{2\pi}{\lambda}a}$${NA} = {\frac{2\pi}{\lambda}a\sqrt{n_{core}^{2} - n_{cladding}^{2}}}$

where λ is the vacuum wavelength, e is the radius of the fibre core, andNA is the numerical aperture. In one embodiment a fibre is said to besingle mode if it has a V number equal to or below 2.40. In oneembodiment a multimode fibre is a fibre which allows two or more modesover a length of the fibre longer than 0.01 m, such as longer than 0.1m, such as longer than 0.5 m, such as longer than 1 m, such as longerthan 10 m, such as longer than 25 m, such as longer than 50 m, such aslonger than 100 m, such as longer than 1 km. In one embodiment the term“allow” also refer to that the arrangement of the fibre allows for twoor more modes. An example of an arrangement which may not allow two ormore modes is a relatively tight coiling of the fibre. In one embodimenta higher order mode is said not be allowed if it is suppressed more than50% relative to the fundamental mode, such as more than 75% relative tothe fundamental mode, such as more than 90% relative to the fundamentalmode, such as more than 10 dB relative to the fundamental mode, such asmore than 20 dB relative to the fundamental mode, such as more than 30dB relative to the fundamental mode, such as more than 40 dB relative tothe fundamental mode, such as more than 50 dB relative to thefundamental mode. In one embodiment supporting at least one higher ordermode refers to supporting 3 or more modes, such as 4 or more modes, suchas 5 or more modes, such as 10 or more modes, such as 25 or more modes,such as 50 or more modes, such as 75 or more modes, such as 100 or moremodes, such as 200 or more modes, such as 500 or more modes, such as1000 or more modes, such as 5000 or more modes, such as 10000 or moremodes. In one embodiment supporting at least one higher order moderefers to supporting less than 10000 modes, such as less than 5000modes, such as less than 1000 modes, such as less than 500 modes, suchas less than 250 modes, such as less than 100 modes, such as less than50 modes, such as less than 25 modes, such as less than 10 modes, suchas less than 5 modes, such as less than 4 modes.

In the event of the pump light having a wide spectrum the generatorfibre is not necessarily multimode for the entire spectrum of the pumplight. Therefore, in one embodiment the generator fibre is multimode forall wavelengths of the pump light. However, in one embodiment thegenerator fibre is multimode for part of the pump light. In oneembodiment the generator fibre is multimoded for all wavelengths forwhich the material forming the core is transparent. In one embodimentsaid generator fibre supports one or more higher order modes in a rangeof wavelength ranging from below 2400 nm, such as below 2200 nm, such asbelow 2000 nm, such as below 1800 nm, such as below 1600 nm, such asbelow 1400 nm such as below 1100 nm. In one embodiment the generatorfibre supports one or more higher order modes in a range of wavelengthsranging from a wavelength above 200 nm, such as above 300 nm, such asabove 500 nm, such as above 600, such as above 700, such as above 800,such as above 900, such as above 1000, such as above 1100, such as above1200, such as above 1300, such as above 1500, such as above 1600, suchas above 1700, such as above 1800, such as above 1900, such as above2000, such as above 2100, such as above 2200.

As discussed above one advantage of the present invention is that thegenerator fibre is in one embodiment able to accept high optical peakpower. The average optical power of the pump light depends on the peakpower as well as the repetition rate and pulse duration of theindividual pulses. In one embodiment the average power of the pump lightinjected into the input end of said generator fibre is arranged to bemore than or equal to 0.1 mW, such as more than or equal to 1 mW, suchas more than or equal to 50 mW, such as more than or equal to 100 mW,150 mW, such as more than or equal to 200 mW, such as more than or equalto 250 mW, such as more than or equal to 300 mW, such as more than orequal to 350 mW, such as more than or equal to 400 mW, such as more thanor equal to 500 mW, such as more than or equal to 1 W, such as more thanor equal to 5 W, such as more than or equal to 10 mW, such as more thanor equal to 50 W, such as more than or equal to 200 W, such as more thanor equal to 500 W, such as more than or equal to 1.000 W, such as morethan or equal to 2.000 W, such as more than or equal to 10.000 W, suchas more than or equal to 50.000 W, such as more than or equal to 100.000W. In one embodiment the average power of the pump light injected intothe input end of said generator fibre is arranged to be less than orequal to 100 kW, such as less than or equal to 50 kW, such as less thanor equal to 10 kW, such as less than or equal to 5 kW, such as less thanor equal to 2 kW, such as less than or equal to 1 kW, such as less thanor equal to 0.5 kW, such as less than or equal to 0.2 kW, such as lessthan or equal to 0.1 kW, such as less than or equal to 50 W, such asless than or equal to 25 W, such as less than or equal to 10 W, such asless than or equal to 5 W, such as less than or equal to 2 W, such asless than or equal to 1 W, such as less than or equal to 500 mW, such asless than or equal to 200 mW, such as less than or equal to 100 mW, suchas less than or equal to 50 mW, such as less than or equal to 25 mW,such as less than or equal to 10 mW, such as less than or equal to 5 mW,such as less than or equal to 1 mW.

In one embodiment the pump light source is arranged to inject pump lightinto the input end of said generator fibre where said pump light issubstantially continuous wave (CW). In the case of CW light the termpeak power otherwise discussed in this text is taken to refer to theaverage optical power of the CW light. Pumping the generator fibre usinga CW source may simplify the optical design relative to a light sourcewith a pulsed pump and some applications of the light source may benefitfrom a CW output. Other advantages may include cost, as high power CWsources may be cheaper pr. average power than pulsed sources. On theother hand, a pulsed pump laser may provide the same peak power as a CWpump with less average power. In one embodiment this provides for a morestabile light source and/or for a light source with a longer life time.In one embodiment a high average optical power of the pump correspondsto a high spectral density of the generated light which may or may notbe desirable depending on the application of the light source.

In one embodiment the pump light source is arranged to inject pump lightinto the input end of said generator fibre, said pump light being pulsedwith a FWHM pulse duration longer than or equal to 1 fs, such as longerthan or equal to 500 fs, such as longer than or equal to 1 ps, such aslonger than or equal to 10 ps, such as longer than or equal to 50 ps,such as longer than or equal to 100 ps, such as longer than or equal to250 ps, such as longer than or equal to 500 ps, such as longer than orequal to 750 ps, such as longer than or equal to 1 ns, such as longerthan or equal to 10 ns, such as longer than or equal to 50 ns, such aslonger than or equal to 100 ns, such as longer than or equal to 500 ns,such as longer than or equal to 1 μs. In one embodiment the pump lightis pulsed with a FWHM pulse duration shorter than or equal to 1 μs, suchas shorter than or equal to 500 ns, such as shorter than or equal to 100ns, such as shorter than or equal to 50 ns, such as shorter than orequal to 10 ns, such as shorter than or equal to 1 ns, such as shorterthan or equal to 750 ps, such as shorter than or equal to 500 ps, suchas shorter than or equal to 250 ps, such as shorter than or equal to 100ps, such as shorter than or equal to 50 ps, such as shorter than orequal to 10 ps, such as shorter than or equal to 1 ps, such as shorterthan or equal to 500 fs, such as shorter than or equal to 250 fs, suchas shorter than or equal to 100 fs, such as shorter than or equal to 50fs, such as shorter than or equal to 1 Ofs, such as shorter than orequal to 5 fs, such as shorter than or equal to ifs. In relation to SCgeneration a pulsed source having short pulse may have the advantage ofproviding a higher peak power with less pulse energy which often is thecritical parameter in relation to damage threshold. A higher peak poweroften provides a wider generated spectrum. On the other hand longerpulses may require a less sophisticated pump source. In one embodimentsaid pump light being pulsed with a FWHM pulse duration is shorter than1 ps, such as shorter than 500 ns, such as shorter than 100 ns, such asshorter than 10 ns, such as shorter than 1 ns, such as shorter than 500ps, such as shorter than 250 ps, such as shorter than 100 ps, such asshorter than 50 ps, such as shorter than 50 ps, such as shorter than 10ps, such as shorter than 5 ps, such as shorter than 1 ps, such asshorter than 500 fs, such as shorter than 100 fs.

In one embodiment the pump source is arranged to inject pulsed lightwhere each pulse comprise a pulse energy higher than 50 μJ, such asequal to or higher than 100 μJ, such as equal to or higher than 200 μJ,such as equal to or higher than 300 μJ, such as equal to or higher than500 μJ, such as equal to or higher than 1 mJ, such as equal to or higherthan 10 mJ.

In one embodiment the output light is pulsed where each pulse comprise apulse energy higher than 50 μJ, such as equal to or higher than 100 μJ,such as equal to or higher than 200 μJ, such as equal to or higher than300 μJ, such as equal to or higher than 500 μJ, such as equal to orhigher than 1 mJ, such as equal to or higher than 10 mJ.

In one embodiment the average peak power density of the pump light overthe core is more than or equal to 100 W/(π·D²/4)≧1, 2 W/μm², such aslarger than or equal to 25 W/μm², such as larger than or equal to 50W/μm², such as larger than or equal to 75 W/μm², such as larger than orequal to 100 W/μm², such as larger than or equal to 250 W/μm², such aslarger than or equal to 500 W/μm², such as larger than or equal to 1kW/μm², such as larger than or equal to 10 kW/μm², such as larger thanor equal to 25 kW/μm², such as larger than or equal to 50 kW/μm², suchas larger than or equal to 100 kW/μm², such as larger than or equal to250 kW/μm². In one embodiment the average peak power density of the pumplight over the core is less than or equal to 250 kW/μm², such as lessthan or equal to 100 kW/μm², such as less than or equal to 10 kW/μm²,such as less than or equal to 1 kW/μm², such as less than or equal to500 W/μm², such as less than or equal to 250 W/μm², such as less than orequal to 100 W/μm², such as less than or equal to 50 W/μm², such as lessthan or equal to 10 W/μm², such as less than or equal to 1 W/μm².

For some applications of a pulsed source with a high average power isbeneficial, e.g. to provide a low signal-to-noise ratio, whereas otherapplications may require a low average power such as to provide eyesafety and/or to reduce cost. Apart from peak power and pulse duration,repetition rate may be used to set the average power. In one applicationit may be advantageous to have high repetition rate so that for a slowerdetector the source will appear to be a CW source. In one embodiment thepump light source is arranged to inject pump light into the input end ofsaid generator fibre, said pump light being pulsed with a repetitionrate higher than or equal to 1 Hz, such as higher than or equal to 500Hz, such as higher than or equal to 1 kHz, such as higher than or equalto 100 kHz, such as higher than or equal to 250 kHz, such as higher thanor equal to 500 kHz, such as higher than or equal to 750 kHz, such ashigher than or equal to 1 MHz, such as higher than or equal to 10 MHz,such as higher than or equal to 50 MHz, such as higher than or equal to100 MHz, such as higher than or equal to 250 MHz, such as higher than orequal to 500 MHz, such as higher than or equal to 1 GHz. In oneembodiment the pump light source is arranged to inject pump light intothe input end of said generator fibre, said pump light being pulsed witha repetition rate lower than or equal to 1 GHz, such as lower than orequal to 500 MHz, such as lower than or equal to 250 MHz, such as lowerthan or equal to 100 MHz, such as lower than or equal to 50 MHz, such aslower than or equal to 25 MHz, such as lower than or equal to 10 MHz,such as lower than or equal to 1 MHz, such as lower than or equal to 500Hz, such as lower than or equal to 250 Hz, such as lower than or equalto 100 Hz, such as lower than or equal to 50 Hz, such as lower than orequal to 10 Hz.

In the context of the present text the spectral width Δλ of the injectedpump light may in one embodiment be the FWHM of the spectrum of theinjected light. However, there may be embodiments of the invention whereFWHM does not provide a good measure of the spectral width Δλ. In oneembodiment the pump light source is arranged to inject light withoptical power at two or more separate center wavelengths substantiallysimultaneously, such as at 1064 nm and 532 nm. In this event thespectral width of the light around each separate center wavelength is tobe treated separately. In one embodiment any claimed limitationinvolving Δλ refers to the limitation being fulfilled for injected lightaround at least one of the center wavelengths, such as for the lightaround at least 50% of the center wavelengths, such as for the lightaround at least the majority of the center wavelengths, such as for thelight around all of the center wavelengths. In one embodiment theinjected pump light is a super continuum. In one such embodiment anyclaimed limitation involving Δλ refers to the limitation being fulfilledfor a spectrum comprising at least 10% of the power of the injectedlight, such as at least 20%, such as at least 30%, such as at least 40%,such as at least 50%, such as at least 60%, such as at least 70%, suchas at least 80%, such as at least 90%, such as at least 99%, such as100%.

In principle the pump source may be any source suitable for providingthe required light parameters, such as pulse properties, beam propertiesand/or CW light. Examples of light sources comprise a Q-switched laser,a mode locked laser, a flash lamp pumped solid state laser, a frequencydoubled laser, a frequency tripled laser, fibre laser, a multimode fibrelaser, side pumped DPSS.

The concept of beam cleanup via Raman scattering (SRS cleanup) refers toan effect where the fundamental mode of a multimode fibre is preferredfor light generated by stimulated Raman scattering. In this wayrelatively more light will be in the fundamental mode after a Ramanshift than before the Raman shift. One model of SRS cleanup may be foundin the paper N. B. Terry, T. G. Alley, T. H. Russell, “An explanation ofSRS beam cleanup in gradedindex fibres and the absence of SRS beamcleanup in step-index fibres”, Opt. Expr., Vol. 15, No. 26, 2007. Inthis paper it was shown that while a graded index-fibre (parabolic corewith α≈2) allowed SRS cleanup a step index fibre did not. It is foundthat other index profiles may also facilitate SRS cleanup. However, inone embodiment the findings of this article should not be construed soas to limit the scope of the present invention. In one embodiment In oneembodiment SRS model cleaning refers to SRS cleanup. In one embodimentof the invention SRS modal cleaning refers to any process where light isred-shifted while relatively more light is found in the fundamental modeafter said red-shifting relative to before.

In one embodiment of the invention it is found SRS model cleaning isallowed for fibres having a core shaped according to the power-law indexprofile where α>2, such as α≧3, such as α≧4, such as α≧5, such as α≧10,such as α≈15, such as α≈20, such as α≈25, such as α≈50.

As discussed above the generator fibre is pumped at wavelengths belowλ_(ZDW) to allow SRS model cleaning to take effect before the smooth SCis generated. In one embodiment the generator is pumped sufficientlyclose to the λ_(ZDW) so that only an insignificant amount of the pumplight is allowed to generate a smooth SC without red shifting, such asless than 20% of the pump light is allowed to generate a smooth SCwithout red shifting, such as less than 10%, such as less than 5%, suchas less than 1%, such as less than 0.01%. In one embodimentλ_(ZDW)−λ_(centre) is equal to or more than 0.5 Raman shift withλ_(centre) as the starting wavelength, such as equal to or more than 1Raman shift, such as equal to or more than 2 Raman shifts, such as equalto or more than 3 Raman shifts, such as equal to or more than 4 Ramanshifts, such as equal to or more than 5 Raman shifts, such as equal toor more than 6 Raman shifts, such as equal to or more than 7 Ramanshifts, such as equal to or more than 8 Raman shift, such as equal to ormore than 9 Raman shifts, such as equal to or more than 10 Raman shifts,such as equal to or more than 15 Raman shift, such as equal to or morethan 20 Raman shifts. In one embodiment λ_(ZDW)−λ_(centre) is equal toor less than 20 Raman shift with λ_(centre) as the starting wavelength,such as equal to or less than 15 Raman shift, such as equal to or lessthan 10 Raman shift, such as equal to or less than 9 Raman shift, suchas equal to or less than 8 Raman shift, such as equal to or less than 7Raman shift, such as equal to or less than 6 Raman shift, such as equalto or less than 5 Raman shift, such as equal to or less than 4 Ramanshift, such as equal to or less than 3 Raman shift, such as equal to orless than 2 Raman shift, such as equal to or less than 1 Raman shift.

In one embodiment the length of the generator fibre may affect theoutput light. In one embodiment the length of the fibre influences towhat extend model clean up takes effect. In one embodiment theabsorption of the fibre affects the output light and in one embodimentone or more non linear process involved in generating the generatedlight is affected by the length of the fibre. The length of the fibremay therefore in one embodiment be considered a design parameter forachieving the desired output light. In one embodiment the generatorfibre has a length of at least 1 m, such as at least 2 m, such as atleast 5 m, such as at least 10 m, such as at least 20 m, such as atleast 50 m, such as at least 75 m, such as at least 100 m, such as atleast 150 m, such as at least 250 m, such as at least 500 m, such as atleast 750 m, such as at least 1.000 m, such as at least 5.000 m, such asat least 10.000 m. In one embodiment the generator fibre has a length ofat less than 10 km, such as less than 1 km, such as less than 500 m,such as less than 250 m, such as less than 100 m, such as less then 75m, such as less than 50 m, such as less than 25 m, such as less than 10,such as less than 5 m, such as less than 1 m. In one embodiment thegenerator fibre has length between 10 m and 100.000 m, such as between50 m and 10.000 m, such as between 100 m and 1.000 m, such as between200 m and 500 m

In one embodiment the generator fibre has a numerical aperture at thecentre wavelength of the pump light of more than or equal to 0.10, suchas more than or equal to 0.12, such as more than or equal to 0.15, suchas more than or equal to 0.18, such as more than or equal to 0.20, suchas more than or equal to 0.22, such as more than or equal to 0.30, suchas more than or equal to 0.40, such as more than or equal to 0.50. Inone embodiment said generator fibre has a numerical aperture at thecentre wavelength of the pump light of less than or equal to 0.50, suchas less than or equal to 0.40, such as less than or equal to 0.30, suchas less than or equal to 0.22, such as less than or equal to 0.20, suchas less than or equal to 0.18, such as less than or equal to 0.15, suchas less than or equal to 0.12, such as less than or equal to 0.10. Asdiscussed above, a relatively large NA may allow the core to accept pumpbeams with relatively high M².

In one embodiment the centre wavelength of the pump source is longerthan or equal to 0.3 μm, such as longer than or equal to 0.4 μm, such aslonger than or equal to 0.5 μm, such as longer than or equal to 0.6 μm,such as longer than or equal to 0.7 μm, such as longer than or equal to0.8 μm, such as longer than or equal to 0.9 μm, such as longer than orequal to 1 μm, such as longer than or equal to 1.1 μm, such as longerthan or equal to 1.2 μm, such as longer than or equal to 1.3 μm, such aslonger than or equal to 1.5 μm, such as longer than or equal to 1.6 μm,such as longer than or equal to 1.7 μm, such as longer than or equal to1.9 μm, such as longer than or equal to 2 μm, such as longer than orequal to 2.1 μm, such as longer than or equal to 2.2 μm. In oneembodiment the centre wavelength of the pump source is shorter than orequal to 2.2 μm, such as shorter than or equal to 2.1 μm, such asshorter than or equal to 2.0 μm, such as shorter than or equal to 1.9μm, such as shorter than or equal to 1.8 μm, such as shorter than orequal to 1.7 μm, such as shorter than or equal to 1.6 μm, such asshorter than or equal to 1.5 μm, such as shorter than or equal to 1.4μm, such as shorter than or equal to 1.3 μm, such as shorter than orequal to 1.2 μm, such as shorter than or equal to 1.1 μm, such asshorter than or equal to 1.0 μm, such as shorter than or equal to 0.9μm, such as shorter than or equal to 0.8 μm, such as shorter than orequal to 0.7 μm, such as shorter than or equal to 0.6 μm, such asshorter than or equal to 0.5 μm, such as shorter than or equal to 0.4μm, such as shorter than or equal to 0.3 μm. In one embodiment a shortcentre wavelength may have the advantage that it takes many red-shiftsto reach the zero dispersion wavelength of the material and thereforemay provide a more clean (i.e. lower M²) red-shifted pump for generationof the smooth super continuum. At the same time pump light with a shortcentre wavelength and its red-shifted derivatives may contribute furtherto the short wavelength section of the output spectrum. This may serveto provide more output power at short wavelengths in the output light.In one embodiment it is preferential to use a longer pump wavelength. Inone such embodiment a longer wavelength provides less red shifted pumplight which may, in one embodiment, otherwise deteriorate the beamquality of output light.

In one embodiment the generator is pumped by at least two or moreseparate centre wavelengths either originating from the same source orfrom separate sources. In one embodiment pump light at two or moreseparate centre wavelengths is pulsed and synchronised, so that thelight overlaps at least partly in some distance through the generatorfibre. This allows in one embodiment the pump light to interact eitherdirectly or via generated light. In one embodiment the light pulsesenter the fibre at the same time and in one embodiment the light pulsesare arranged to meet during the passing of the generator fibre. In oneembodiment the pulses are arranged to meet substantially at the outputend of the generator fibre, so as to avoid substantially all interactionin the non-linear generation while obtaining a single pulse on theoutput of the generator fibre. In one embodiment the light pulses do notoverlap in the generator fibre. In one embodiment light generated due tonon-linear effects in the fibre from one pulse is arranged to overlapwith a second pulse and/or its derivatives (i.e. light generated fromthat pulse due to non-linear effects in the fibre). As noted above oneor more pulses may originate from the same source, such as one pulse at1064 nm and a second which is part of the first pulse frequency doubledto 532 nm. When pulses originate from the same source phase matchingbetween the pulses may be simpler to obtain relative to synchronisingmultiple sources. However, phase matching is in one embodiment notnecessarily required for non-linear interaction, such as in embodimentswhere pump light is provided as described by G. Genty, J. M. Dudley, B.J. Eggleton, “Modulation control and spectral shaping of optical fibresupercontinuum generation in the picosecond regime”, Appl Phys B (2009)94:187-194.

In the present context separate centre wavelengths are in one embodimenttaken to mean that the spectrum of the injected pump light comprises twoor more peaks which overlap less than 50%, such as less than 40%, suchas less than 30%, such as less than 20%, such as less than 10%, such asless than 1%. In one embodiment separate centre wavelengths areunderstood to mean that the pump source(s) is designed so that theinjection of pump light at one or more separate wavelengths may, atleast in principle, be switched off e.g. by blocking a beam path in thepump source, substantially without affecting the other(s).

In one embodiment the pump source is formed by multiple light sourcessuch as a laser diode array, multiple lasers combined via a fibre opticcombiner, and multiple lasers combined, either coherently orincoherently, via bulk optics. These multiple sources may form a pumpsource having a single centre wavelength or a pump source with two ormore separate centre wavelengths. As discussed above, the required beamquality of the pump source may in one embodiment be relatively low andit may therefore be relatively simple to obtain high optical power inthe injected pump light by combining several sources.

In one embodiment the injected pump light has a high beam quality. Inone embodiment M² _(in) of the pump light injected on the input end ofthe generator fibre is lower than or equal to 500, such as lower than orequal to 250, such as lower than or equal to 250, such as lower than orequal to 100, such as lower than or equal to 80, such as lower than orequal to 60, such as lower than or equal to 40, such as lower than orequal to 25, such as lower than or equal to 10, such as lower than orequal to 4, such as lower than or equal to 2, such as lower than orequal to 1.7, such as less than 1.3, such as less than 1.2, such assubstantially 1.1, such as less than 1.1, such as substantially 1. Inone embodiment In one embodiment the pump source is a single modesource. In one embodiment the generator may accept pump light with arelatively high M² _(in), e.g. by having a relatively large corediameter and/or due to SRS modal cleaning (or other means of modalcleaning) the injected pump light may be allowed to have an M² _(in)that is higher than or equal to 1, such as such as higher than or equalto 1.1, such as such as higher than or equal to 1.3, such as higher thanor equal to 4, such as higher than or equal to 10, such as higher thanor equal to 25, such as higher than or equal to 40, such as higher thanor equal to 60, such as higher than or equal to 80, such as higher thanor equal to 100, such as higher than or equal to 250, such as higherthan or equal to 500. In one embodiment a high M² _(in) may bebeneficial as high optical power may be provided with a relatively cheapand/or simple and/or robust source.

As explained above the SRS modal cleaning may in one embodiment improvethe beam quality of the output light significantly. Accordingly, in oneembodiment said pump source and said generator fibre are arranged sothat during use said generator fibre emits output light at the outputend having an average M² _(out) of less than or equal to 10, such asless than or equal to 5, such as less than or equal to 4, such as lessthan or equal to 3, such as less than or equal to 2, such as less thanor equal to 1.3, such as less than or equal to 1.1, such assubstantially 1. In one embodiment the average M² _(out) is more than orequal to 1, such as higher than or equal to 1.1, such as higher than orequal to 1.2, such as more than or equal to 1.3, such as more than orequal to 2, such as more than or equal to 3, such as more than or equalto 4, such as more than or equal to 5, such as more than or equal to 10.Here the average M² _(out) refers to an average over wavelengths in thespectrum of the output light. In one embodiment this average is aweighted average so the wavelengths having high optical power attributemore to the average than wavelengths with low optical power. In oneembodiment the spectrum substantially around the centre wavelength(s) ofthe pump source is excluded from the calculation of the average, heresubstantially around being taken to mean more than 0.5 nm on both sidesof said centre wavelength, such as more than 1 nm on both sides of saidcentre wavelength, such as more than 10 nm on both sides of said centrewavelength, such as more than 20 nm on both sides of said centrewavelength, such as more than 100 nm on both sides of said centrewavelength. In one embodiment substantially around is taken to mean lessthan 100 nm on both sides of said centre wavelength, such as less than100 nm on both sides of said centre wavelength, such as less than 100 nmon both sides of said centre wavelength, such as less than 20 nm on bothsides of said centre wavelength, such as less than 10 nm on both sidesof said centre wavelength, such as less than 1 nm on both sides of saidcentre wavelength.

In one embodiment the output light is substantially diffraction limitedat least over the majority of the spectrum.

As explained the SRS modal cleaning may improve the beam quality of thegenerated output light relative to the injected pump light. Accordinglyin one embodiment M² _(out)/M² _(in) is less than one. However, in oneembodiment a pump source is applied which is capable of providinginjected light with a low M² _(in). Due to the multimoded nature of thegenerator fibre the injected light may in such an embodiment bescrambled to also reside in higher order modes inside the fibre. Forsuch cases M² _(out)/M² _(in) may be higher than one. Therefore, inembodiment the pump source and the generator fibre is arranged so thatduring use said generator fibre emits output light at the output endhaving an average M² _(out) and said pump source injects pump lighthaving an M² _(in) wherein M² _(out)/M² _(in) is less than or equal to5, such as less than or equal to 2, such as less than or equal to 1,such as less than or equal to 0.1, such as less than or equal to 0.01,such as less than or equal 0.005, such as less than or equal to 0.002.In one embodiment M² _(out)/M² _(in) is more than or equal to 0.002,such as more than or equal to 0.005, such as more than or equal to 0.01,such as more than or equal to 0.1, such as more than or equal to 1, suchas more than or equal 2, such as more than or equal to 5. In oneembodiment M² _(out)/M² _(in) should be calculated as a function ofwavelength or over specific wavelength range. In one embodiment thisonly refers to the calculation of the average M² _(out). In oneembodiment average M² _(out) is calculated as discussed above e.g. withexclusion of wavelengths around the centre wavelength of the pump light.In one embodiment the average M² _(out) is calculated for allwavelengths of the spectrum longer than λ_(ZDW).

BRIEF DESCRIPTION OF DRAWINGS

The invention will be illustrated further below in connection withexemplary embodiments and with reference to the drawings in which:

FIG. 1 shows a schematic presentation of a light source according to theinvention,

FIG. 2 shows a the spectrum of the output light produced by a lightsource according to the invention,

FIG. 3 shows a schematic depiction of a light source according to theinvention applied to LIDAR,

FIG. 4 shows a schematic presentation of a light source according to theinvention applied to eye safe LIDAR,

FIG. 5 shows a schematic presentation of a light source according to theinvention applied to seed an optical fibre amplifier,

FIG. 6 shows a schematic presentation of a light source according to theinvention applied to form a tunable light source,

FIG. 7 shows examples of refractive index profiles.

The figures are schematic and may be simplified for clarity. Throughout,the same reference numerals are used for identical or correspondingparts.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the examples are given by way of illustration.Furthermore it should be noted that a feature discussed in relation toone embodiment or example is not limited to this specific embodiment butmay be applied in other embodiments as well.

EXAMPLE 1

FIG. 1 show a schematic outline of a super continuum light source 11according to the invention. The pump source 12 emits a pump beam 13which is focused onto the input end 16 of the generator fibre 15 via alens 14. At the output end 17 of the generator fibre 15 the output light18 is emitted. In one embodiment the output light is substantiallysingle mode at least for most wavelengths of the generated SC asdiscussed above. An exemplary spectrum of the output light of a lightsource according to the invention is shown in FIG. 2. Here acommercially available fibre (Infinicor of Corning Inc, Corning N.Y.,USA) is applied as generator fibre 15. This fibre is a graded indexfibre having a substantially parabolic index profile with a quoted coresize with a diameter of 50 μm. The generator fibre 15 is pumped by apulsed laser at 1064 nm at relatively high peak power (˜200 uJ pulseenergy, ˜2 ns pulse, ˜100 kW peak power and M² of about 2). The residualpower from the pump light is identifiable as the peak 21. The peaks 22to 25 may be attributed to subsequent Ramen shifting of the pump lightas these correspond well to the stokes lines calculated for silica witha 1064 pump: S₁=1115 nm, S₂=1173 nm, S₃=1235 nm, S₄=1305. As the fourthStokes shift passes the zero dispersion line of a silica fibre thislight then acts as the pump for generation of the smooth SC likelyprovided by four wave mixing (FWM) providing a spectrum extending fromroughly half the zero dispersion wavelength to the long wavelengthabsorption edge of the fibre (not shown). FWM likely halts further Ramanshifts since the gain of FWM is higher. Due to SRS modal cleaning theRaman shifted light has a relatively high beam quality and as this issubstantially maintained the light source capable of providing asubstantially single mode output at most wavelengths. As discussed abovethe pump light may comprise one or more higher order modes and mayincrease the M² _(out) at certain wavelengths. In one embodiment theoutput light of a SC light source according to the invention issubsequently spatially filtered to eliminate optical components in theoutput light belonging to higher order modes and/or residual pump light.

EXAMPLE 2

In this example the light source is identical to that of example 1except that the pump source inject pump light at 1064 nm and 532 nmsimultaneously. In one embodiment Raman shifted light originating fromthe 532 nm pump merges with the short wavelength light generated fromthe 1064 nm pump thus providing a continuum with greater spectral width.In this way a source may be provided which provides a larger portion ofenergy in the visible range.

EXAMPLE 3

FIG. 3 shows a schematic depiction of a light source according to theinvention applied in a LIDAR system 31. A telescope 32 comprising alight source according to the invention emits a SC beam 33 and detectsreflected light from aerosols 34 in the atmosphere. The reflected lightis analysed in a spectrometer 35 and results are relayed to a computer36. Due to the high optical power which may be provided by an embodimentof the invention the signal-to-noise ratio (SNR) may be improved whichmay effectively extend the range of the system and/or allow fordetection of smaller amounts of aerosols. Similarly to this example thesystem may instead be configured as a transmission system where thelight beam is directed towards a detector. Such a system is alsosometimes referred to as a DOAS system.

The present invention may also be applied as a light source in so calledactive hyperspectral imaging where the increased power and/or good beamquality which may be provided with the present invention may improve SNRof such systems. Further details of active hyperspectral imaging mayfound in the literature such as the paper: M. L. Nischan, R. M. Joseph,J. C. Libby, and J. P. Kerekes, “Active Spectral Imaging”, Lincoln Lab.Journ., Vol. 14, p. 131 pp, 2003.

EXAMPLE 4

FIG. 4 shows a schematic outline of a light source 41 according to theinvention where the output light is directed to a filter 42. In oneembodiment the filter is arranged to reduce transmitted light atwavelengths shorter than about 1500 nm. Wavelengths above roughly 1500nm are typically considered eye-safe because the light is nottransmitted by the lens of the eye. The present invention provides asimple and robust technique for generating high energy light in thisregion of the spectrum. When compared to other techniques such as OPO'sthe invention may in one embodiment have lower cost, higher stabilityand/or be more compact.

EXAMPLE 5

FIG. 5 shows a schematic outline of an optical amplifier 51 comprising alight source according to the invention 11 as a seed for a gain medium52. The gain medium 52 is shown as an active fibre but may in principlebe any suitable active optical medium such as a suitable semi conductor.In one embodiment the output light of the light source 11 is filtered bythe filter 53. With a broad spectrum it may be possible to selectspecific wavelength(s) to pump the gain medium. Such wavelengths mayprovide desired amplifier performance such as low noise.

EXAMPLE 6

FIG. 5 shows a schematic outline of a tuneable light source 61comprising a light source according to the invention 11. The lightsource 61 further comprises a non linear crystal 62 which may be appliedto transform wavelengths of the output light 18 of the SC light source11 for example to shorter wavelengths depending on the angle ofincidence on the crystal 62. In this way the light source 61 may betuneable by changing the angle between output light 18 and crystal 62.

EXAMPLE 7

FIGS. 7 (a-f) shows exemplary refractive index profiles of arotationally symmetric optical fibre. The core 71 may be said toresemble the core shape of a graded index profile in FIGS. 7 a, b,dwhereas the core 72 of FIGS. 7 c,e resemble the core shape of a stepindex fibre. The region 77 of FIG. 7 f represents what is typicallyknown as a cladding pedestal to offset the index of a core 72. Therefractive index of cladding 73 is shown as a constant value surroundingany index variation around the core. In a real world fibre therefractive index of the cladding 73 will not extend to infinity butnormally to a diameter large enough to enstJre that for all practicalpurposes the outer diameter of the cladding is irrelevant for lightguided in the core. In principle the refractive index profile may not beconstant as discussed in regard to the characteristic diameter D. Yetanother definition of the characteristic diameter of the core of a fibreis the outer diameter of a refractive index profile which may provideguiding when light is launched in the center of the fibre (assumingcylinder symmetry). In such an embodiment a guided field may have anevanescent field in the cladding. This characteristic diameter has beenindicated by the double arrow 74 in each figure. FIGS. 7 b, d, f has aso-called depressed cladding 75 surrounding the core. The profile 7 dfurther has an outer core 76 which may guide light at longer wavelengthswhereas the core 71 functions as the primary guide (in one embodimentunderstood as comprising substantially all the optical energy of themode within its bounds) for shorter wavelengths. In one embodiment thecharacteristic diameter D is defined as the diameter of the inner corewhen the fibre has multiple core regions. In one embodiment D is definedas the characteristic diameter of the dominant core at the centerwavelength of the pump and/or the zero dispersion wavelength.

FIG. 8 shows an index profile of a preform suitable for drawing a gradedindex fibre. The profile was obtained by means of plasma deposition andexhibits a central dip due to the cavity in the glass tube to whichinside the plasma has been deposited. Such performs are commercializede.g. by the company Draka Fibre Inc. Examples of MM fibres that may finduse for the present invention includes commercially available fibres,such as the following fibres from Draka Fibre Inc.:

-   -   Standard 50/125 m and 62.5/125 m (OM1 and OM2)    -   iCap and HiCapXS (FTTH); 50/125 m and 62.5/125 m (1 Gb/s OM1+        and OM2+)    -   MaxCap300 and MaxCap550 (10 Gb/s OM3 and OM3+).

In the present text the invention has been discussed with basis in supercontinuum generation in silica based optical fibre. However, theinvention may also be practiced in other types of waveguides. Thesewaveguides include planar waveguides and waveguides or fibres having abase material different from silica.

1-46. (canceled)
 47. A supercontinuum light source comprising a pumpsource arranged to emit pump light in a pump beam, said pump lighthaving a centre wavelength λ_(centre), the light source furthercomprising a generator fibre having an input end and an output end, saidgenerator fibre supporting a fundamental mode and one or more higherorder modes at least at said centre wavelength, and having a zerodispersion wavelength λ_(ZDW) for said fundamental mode and a transverserefractive index profile with a core region having a characteristicdiameter D≧10 μm, said pump source being arranged to inject pump lightinto the input end of said generator fibre with an optical peak powerP≧250 W and a spectral width Δλ spectrum, wherein said refractive indexprofile is arranged to allow SRS modal cleaning andλ_(ZDW)−(λ_(centre)+Δλ/2)≧0.
 48. The light source of claim 47 whereinsaid characteristic diameter D is larger than or equal to 25 μm.
 49. Thelight source of claim 47 wherein said characteristic diameter D islarger than or equal to 50 μm.
 50. The light source of claim 47 whereinsaid characteristic diameter D is larger than or equal to 100 μm. 51.The light source of claim 47 wherein said generator fibre has anumerical aperture of more than or equal to 0.10.
 52. The light sourceof claim 47 wherein said generator fibre has a numerical aperture ofmore than or equal to 0.20.
 53. The light source of claim 47 whereinsaid generator fibre has a numerical aperture of more than or equal to0.40.
 54. The light source of claim 47 wherein λ_(ZDW)−λ_(centre) isequal to or more than 0.5 Raman.
 55. The light source of claim 47wherein λ_(ZDW)−λ_(centre) is equal to or more than 5 Raman shift. 56.The light source of claim 47 wherein λ_(ZDW)−λ_(centre) is equal to orless than Raman shift.
 57. The light source of claim 47 wherein theaverage power of the pump light injected into the input end of saidgenerator fibre is arranged to be more than or equal to 1 W.
 58. Thelight source of claim 47 wherein said pump light source is arranged toinject pump light with optical power at two or more separate centrewavelengths substantially simultaneously.
 59. The light source of claim47 wherein the pump source is arranged so that M² _(in) of the pumplight injected on the input end of the generator fibre is higher than orequal to
 4. 60. The light source of claim 47 wherein the pump source isarranged so that M² _(in) of the pump light injected on the input end ofthe generator fibre is higher than or equal to
 25. 61. The light sourceof claim 47 wherein said pump source and said generator fibre arearranged so that during use said generator fibre emits output light atthe output end having an average M² _(out) and said pump source injectspump light having an M² _(in) wherein M² _(out)/M² _(in) is less than orequal to
 1. 62. The light source of claim 47 wherein said pump sourceand said generator fibre are arranged so that during use said generatorfibre emits output light having spectral width of more than 500 nm. 63.The light source of claim 47 wherein said pump source and said generatorfibre are arranged so that during use said generator fibre emits outputlight having spectral width of more than 1000 nm.
 64. The light sourceof claim 47 wherein said generator fiber is a graded index fiber. 65.The light source of claim 47 wherein the cores of said generator fiberhas a parabolic index profile.
 66. An optical system comprising a lightsource according to claim
 47. 67. A tunable light source comprising asuper continuum light source according claim 47 and a non-linear crystalarranged so that at least a part of output light emitted from the outputend of the generator fibre is brought into interaction with saidnon-linear crystal under an angle φ relative to the surface of saidnon-linear crystal.
 68. A system selected from the group of ahyperspectral imaging system, white light LIDAR (DOAS) system, anoptical amplifier and an illumination system, wherein the systemcomprises the supercontinuum light source of claim 47.